gcc: remove multilibs for -mfdiv/-mno-fdiv

There are a few reasons for this.  First, the ABI requires that these
instructions be emulated if not implemented in HW.  Second, the userspace
emulation library wasn't markedly faster than the kernel's.  Third, the
multilib explosion was becoming intractible.

Ultimately, the plan is to replace illegal instructions with jumps to
userspace emulation routines, which will achieve reasonable performance
portably (without multilibs).  This support is forthcoming.

gcc/gcc/config/riscv/t-elf

@@ -1,5 +1,4 @@
 # Build the libraries for both hard and soft floating point
 
-MULTILIB_OPTIONS = msoft-float m64/m32 mfdiv mno-atomic
-MULTILIB_DIRNAMES = soft-float 64 32 fdiv no-atomic
-MULTILIB_EXCEPTIONS = *msoft-float*/*mfdiv*
+MULTILIB_OPTIONS = msoft-float m64/m32 mno-atomic
+MULTILIB_DIRNAMES = soft-float 64 32 no-atomic

gcc/gcc/config/riscv/t-linux64

@@ -1,6 +1,5 @@
 # Build the libraries for both hard and soft floating point
 
-MULTILIB_OPTIONS = m64/m32 msoft-float mfdiv mno-atomic
-MULTILIB_DIRNAMES = 64 32 soft-float fdiv no-atomic
-MULTILIB_EXCEPTIONS = *msoft-float*/*mfdiv*
+MULTILIB_OPTIONS = m64/m32 msoft-float mno-atomic
+MULTILIB_DIRNAMES = 64 32 soft-float no-atomic
 MULTILIB_OSDIRNAMES = ../lib ../lib32