[RISC-V] Adjust recently added test

[RISC-V] Adjust recently added test

The recently added zbb-sext test includes stdint.h and explicitly asks for the
lp64 abi (not lp64d!).

This will fail on a native riscv system as the system headers don't support
lp64 -- they assume "d" is included.

It looks like most tests are including stdint-gcc instead of stdint.  Not a fan
of that, but it seems to be how we've been handling this kind of issue to-date.

gcc/testsuite
	* gcc.target/riscv/zbb-sext.c: Include stdint-gcc.h instead of
	stdint.h.