libphobos: RISC-V: Fix soft-float build errors with IEEE exception flags

libphobos: RISC-V: Fix soft-float build errors with IEEE exception flags

Fix assembly errors:

.../libphobos/src/std/math.d: Assembler messages:.../libphobos/src/std/math.d:4773: Error: unrecognized opcode `frflags a0'.../libphobos/src/std/math.d:4856: Error: unrecognized opcode `fsflags a5'.../libphobos/src/std/math.d:4856: Error: unrecognized opcode `fsflags a5'.../libphobos/src/std/math.d:4773: Error: unrecognized opcode `frflags a0'.../libphobos/src/std/math.d:5549: Error: unrecognized opcode `fscsr a5'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a5'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a5'.../libphobos/src/std/math.d:5549: Error: unrecognized opcode `fscsr a5'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a5'.../libphobos/src/std/math.d:5549: Error: unrecognized opcode `fscsr a0'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a0'.../libphobos/src/std/math.d:5456: Error: unrecognized opcode `frcsr a0'.../libphobos/src/std/math.d:5549: Error: unrecognized opcode `fscsr s2'make[8]: *** [Makefile:1119: std/math.lo] Error 1

triggered with the RISC-V lp64 multilib in a GCC build configured with
`--enable-multilib --enable-languages=all --target=riscv64-linux-gnu'.
This is due to unconditional explicit use of F extension instructions
within inline assembly, to access IEEE exception flags.  The use of
these instructions is not allowed when building for a soft-float ABI.

Correct the problem by wrapping said inline assembly into a conditional
such that if `D_SoftFloat' is true, then reads from IEEE exception flags
return 0 and writes are ignored instead, complementing r270522
("libphobos: Add D support for RISC-V Linux"), which is an updated
version of <https://gcc.gnu.org/ml/gcc-patches/2019-04/msg00325.html>,
where the problematic code has originated from.

libphobos/ChangeLog:

2019-05-02  Maciej W. Rozycki  <macro@wdc.com>

	* std/math.d (IeeeFlags.getIeeeFlags): Handle RISC-V soft-float ABI.
	(IeeeFlags.resetIeeeFlags): Likewise.
	(FloatingPointControl.getControlState): Likewise.
	(FloatingPointControl.setControlState): Likewise.

From-SVN: r270815