doc(format): system.nim: doc of hostCPU for loongarch64

In doc, `loongarch64` used to be written as `'"loongarch64"'`

since it's [supported](nim-lang#19223)

Author: litlighilit

lib/system.nim

@@ -1037,7 +1037,7 @@ const
     ## Possible values:
     ## `"i386"`, `"alpha"`, `"powerpc"`, `"powerpc64"`, `"powerpc64el"`,
     ## `"sparc"`, `"amd64"`, `"mips"`, `"mipsel"`, `"arm"`, `"arm64"`,
-    ## `"mips64"`, `"mips64el"`, `"riscv32"`, `"riscv64"`, '"loongarch64"'.
+    ## `"mips64"`, `"mips64el"`, `"riscv32"`, `"riscv64"`, `"loongarch64"`.
 
   seqShallowFlag = low(int)
   strlitFlag = 1 shl (sizeof(int)*8 - 2) # later versions of the codegen \