env/hardening: fix hardening on MIPS.

Thanks to @chneukirchen for finding the correct solution:

gcc sets -mno-shared by default when compiling non-PIC, and because
we are overriding the builtin specs, this internal rule set for gnu/mips
does not trigger:

gcc/config/mips/gnu-user.h:/* Default to -mno-shared for non-PIC.  */
gcc/config/mips/gnu-user.h:  " %{mshared|mno-shared|fpic|fPIC|fpie|fPIE:;:-mno-shared}"

So that we now use a specific specs file just for mips that sets -mshared for PIC.

This fixes building packages with hardening enabled for MIPS.

common/environment/configure/gccspecs/hardened-mips-cc1

@@ -0,0 +1,8 @@
+*cpp_options:
++ %{!fpie:%{!fPIE:%{!fpic:%{!fPIC:%{!fno-pic:-fPIE -mshared}}}}}
+
+*cc1_options:
++ %{!fpie:%{!fPIE:%{!fpic:%{!fPIC:%{!fno-pic:-fPIE -mshared}}}}}
+
+*asm_options:
++ %{!fpie:%{!fPIE:%{!fpic:%{!fPIC:%{!fno-pic:-mshared}}}}}