c++/modules: Fallback to ftruncate if posix_fallocate fails [PR115008]

Depending on the libc and filesystem, in cases where posix_fallocate
cannot do an efficient preallocation it may return EINVAL.  In such a
case we should fall back to ftruncate instead.

Apparently, depending on the system the use of posix_fallocate can have
a noticeable speedup over ftruncate in general (depending on the system)
so it probably isn't worth it to use ftruncate in all cases.

	PR c++/100358
	PR c++/115008

gcc/cp/ChangeLog:

	* module.cc (elf_out::create_mapping): Fallback to ftruncate if
	posix_fallocate fails.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>