gas: pru: Fix trailing whitespace handling

With commit 6ae8a30d, arguments followed
by a C-style comment ended up with a trailing space.  That extra space
character confused the PRU register name matching, leading to spurious
errors about unrecognized registers.  This affected existing code like
newlib's setjmp.s for pru.

Fix by stripping the trailing whitespace for any argument.

Even with 6ae8a30d reverted, this patch
is safe to be applied.

Successfully regression-tested with GCC and newlib testsuites for pru-unknown-elf.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>