Implement round, floor, ceil on the LLVM backend

[jiribenes]

Cherry-picked from #542.

I don't really know why the tests fail on CI: the intrinsics are there from at least LLVM 12, and it does work locally...
The CI fails failed with:

/usr/bin/ld: ./out/tests/effekt.llvmtests/doubles.o: in function `round_1113':
doubles.ll:(.text+0xff2): undefined reference to `round'
/usr/bin/ld: ./out/tests/effekt.llvmtests/doubles.o: in function `floor_1118':
doubles.ll:(.text+0x1002): undefined reference to `floor'
/usr/bin/ld: ./out/tests/effekt.llvmtests/doubles.o: in function `ceil_1120':
doubles.ll:(.text+0x1012): undefined reference to `ceil'
collect2: error: ld returned 1 exit status

See below for the resolution.

[jiribenes]

Oops, I thought we were linking -lm already!

[jiribenes]

cc @phischu, do you foresee any problems with linking in -lm in the LLVM backend?
This is only relevant when using gcc, it works fine without it on clang.

[phischu]

Not an actual problem, just that this is the beginning of a never ending stream of C libraries we are linking against.

[phischu]

There could be subtle differences between the backends, but I don't care.

[phischu]

Not an actual problem, just that this is the beginning of a never ending stream of C libraries we are linking against.