Add mips64 and mips64el CPU platforms (#5866)

lib/posix/posix_other.nim

@@ -572,7 +572,8 @@ else:
     MAP_POPULATE*: cint = 0
 
 when defined(linux) or defined(nimdoc):
-  when defined(alpha) or defined(mips) or defined(parisc) or
+  when defined(alpha) or defined(mips) or defined(mipsel) or
+      defined(mips64) or defined(mips64el) or defined(parisc) or
       defined(sparc) or defined(nimdoc):
     const SO_REUSEPORT* = cint(0x0200)
       ## Multiple binding: load balancing on incoming TCP connections

lib/system.nim

@@ -1313,7 +1313,7 @@ const
   hostCPU* {.magic: "HostCPU".}: string = ""
     ## a string that describes the host CPU. Possible values:
     ## "i386", "alpha", "powerpc", "powerpc64", "powerpc64el", "sparc",
-    ## "amd64", "mips", "mipsel", "arm", "arm64".
+    ## "amd64", "mips", "mipsel", "arm", "arm64", "mips64", "mips64el".
 
   seqShallowFlag = low(int)
 

lib/system/platforms.nim

@@ -24,6 +24,8 @@ type
     amd64,                     ## x86_64 (AMD64); 64 bit x86 compatible CPU
     mips,                      ## Mips based processor
     mipsel,                    ## Little Endian Mips based processor
+    mips64,                    ## 64-bit MIPS processor
+    mips64el,                  ## Little Endian 64-bit MIPS processor
     arm,                       ## ARM based processor
     arm64,                     ## ARM64 based processor
     vm,                        ## Some Virtual machine: Nim's VM or JavaScript
@@ -73,6 +75,8 @@ const
                elif defined(amd64): CpuPlatform.amd64
                elif defined(mips): CpuPlatform.mips
                elif defined(mipsel): CpuPlatform.mipsel
+               elif defined(mips64): CpuPlatform.mips64
+               elif defined(mips64el): CpuPlatform.mips64el
                elif defined(arm): CpuPlatform.arm
                elif defined(arm64): CpuPlatform.arm64
                elif defined(vm): CpuPlatform.vm