MIPS: Allow FP support to be disabled

Allow the user to configure the kernel to omit support for floating
point, by setting CONFIG_MIPS_FP_SUPPORT=n. In an attempt to avoid
problems for users who don't understand the impact of this, only expose
the option when CONFIG_EXPERT=y.

When CONFIG_MIPS_FP_SUPPORT=n all support for FPU hardware, FPU
emulation & FP context will be removed from the kernel. If a userland
program attempts to execute a floating point instruction it will receive
a SIGILL.

Setting CONFIG_MIPS_FP_SUPPORT=n shaves around 112KB from a
64r6el_defconfig build using GCC 8.1.0.

This also helps prepare us for supporting the nanoMIPS ISA, for which
floating point support has not been finalized.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21014/
Cc: linux-mips@linux-mips.org

arch/mips/Kconfig

@@ -2256,7 +2256,19 @@ config CPU_GENERIC_DUMP_TLB
 	default y if !(CPU_R3000 || CPU_R8000 || CPU_TX39XX)
 
 config MIPS_FP_SUPPORT
-	def_bool y
+	bool "Floating Point support" if EXPERT
+	default y
+	help
+	  Select y to include support for floating point in the kernel
+	  including initialization of FPU hardware, FP context save & restore
+	  and emulation of an FPU where necessary. Without this support any
+	  userland program attempting to use floating point instructions will
+	  receive a SIGILL.
+
+	  If you know that your userland will not attempt to use floating point
+	  instructions then you can say n here to shrink the kernel a little.
+
+	  If unsure, say y.
 
 config CPU_R2300_FPU
 	bool