feat: FFI_PORTABLE to build a maximally portable binary

Ref: filecoin-project/lotus#12423

Builds using the same flags as rust/scripts/build-release.sh does when called
from CI to build the release bundle.

README.md

@@ -34,6 +34,14 @@ rm .install-filcrypto \
     ; FFI_BUILD_FROM_SOURCE=1 FFI_USE_BLST_PORTABLE=1 make
 ```
 
+Alternatively, to build a maximally portable binary without optimizing for the current CPU, set `FFI_PORTABLE=1`:
+
+```shell
+rm .install-filcrypto \
+    ; make clean \
+    ; FFI_BUILD_FROM_SOURCE=1 FFI_PORTABLE=1 make
+```
+
 By default, a 'gpu' option is used in the proofs library. This feature is also used in FFI unless explicitly disabled. To disable building with the 'gpu' dependency, set `FFI_USE_GPU=0`:
 
 ```shell
@@ -44,7 +52,7 @@ rm .install-filcrypto \
 
 #### GPU support
 
-CUDA for GPU support is now enabled by default in the proofs library. This feature can optionally be replaced by OpenCL by using `FFI_USE_OPENCL=1` set in the environment when building from source. Alternatively, if the CUDA toolkit (such as `nvcc`) cannot be located in the environment, OpenCL support is used instead. To disable GPU support entirely, set `FFI_USE_GPU=0` in the environment when building from source.
+CUDA for GPU support is now enabled by default in the proofs library. This feature can optionally be replaced by OpenCL by using `FFI_USE_OPENCL=1` set in the environment when building from source. Using `FFI_PORTABLE=1` will also enable OpenCL support. Alternatively, if the CUDA toolkit (such as `nvcc`) cannot be located in the environment, OpenCL support is used instead. To disable GPU support entirely, set `FFI_USE_GPU=0` in the environment when building from source.
 
 There is experimental support for faster C2 named "SupraSeal". To enable it, set `FFI_USE_CUDA_SUPRASEAL=1`. It's specific to CUDA and won't work with OpenCL.
 

install-filcrypto

@@ -190,7 +190,7 @@ build_from_source() {
         if [ "${FFI_USE_GPU}" == "0" ]; then
             gpu_flags=""
         # Check if OpenCL support is specified or we're building on Darwin.
-        elif [ "${FFI_USE_OPENCL}" == "1" ] || [ "$(uname -s)" = "Darwin" ]; then
+        elif [ "${FFI_USE_OPENCL}" == "1" ] || [ "${FFI_PORTABLE}" == "1" ] || [ "$(uname -s)" = "Darwin" ]; then
             gpu_flags=",opencl"
         else
             # If GPUs are enabled and SupraSeal is not, default to CUDA support
@@ -226,7 +226,7 @@ build_from_source() {
 
     additional_flags=""
     # Add feature specific rust flags as needed here.
-    if [ "${FFI_USE_BLST_PORTABLE}" == "1" ]; then
+    if [ "${FFI_USE_BLST_PORTABLE}" == "1" ] || [ "${FFI_PORTABLE}" == "1" ]; then
         additional_flags="${additional_flags} --no-default-features --features ${use_multicore_sdr},blst-portable${gpu_flags}${use_fixed_rows_to_discard}"
     else
         additional_flags="${additional_flags} --no-default-features --features ${use_multicore_sdr}${gpu_flags}${use_fixed_rows_to_discard}"
@@ -243,6 +243,11 @@ build_from_source() {
 }
 
 get_release_flags() {
+    if [ "${FFI_PORTABLE}" == "1" ]; then
+        echo ""
+        return
+    fi
+
     local __features=""
 
     # determine where to look for CPU features