MLECO-6389: Including cmath header in math library

Fix for some third party toolchains failing to compile the
math library without explicitly including the cmath header.

A trivial non-related doumentation fix to say that LLVM or
Clang based toolchains may not work for ExecuTorch either.
Currently, only Arm GNU Embedded Toolchain works with this
flow.

Change-Id: I12889d796886fb0532002c01b1b603982c926e31
Signed-off-by: Kshitij Sisodia <kshitij.sisodia@arm.com>