Description:
Related to https://sourceware.org/bugzilla/show_bug.cgi?id=30551

In summary, using the provided Makefile to compile BE and LE versions of the provided test.s, doing objdump then objcopy to strip it to a raw binary and running xxd on both binaries, we can see the following when we perform diff on it:

diff le_test.hex be_test.hex
1c1
< 00000000: 6f00 0000 0000 0000 cdcc 8c3f 0000 0000  o..........?....
---

> 00000000: 6f00 0000 0000 0000 cccd 3f8c 0000 0000  o.........?.....

The upper byte sequence is properly encoded as LE, while the lower byte sequence is not a proper BE encoding: the byte swap only happened at 2 byte boundaries. The proper BE sequence in this case is: 3f 8c cc cd

Changes Made:
Inside gas/config/tc-riscv.c inside function md_atof, ieee_md_atof is called with macro constant TARGET_BYTES_BIG_ENDIAN which is always 0 if not defined otherwise. This ignores -mbig-endian directive. Instead, use a global vairable target_big_endian that is set depending on presence of the -mbig-endian directive.

Testing:
After the above change, building the provided test.s via the provided Makefile and diffing the resulting binaries gives the following:
diff le_test.hex be_test.hex

< 00000000: cdcc8c3f 00000000                    ...?....
---

> 00000000: 3f8ccccd 00000000                    ?.......