Feat: Implement support for 'riscv64-elf-gcc' toolchain

This commit introduces logic to detect and utilize the 'riscv64-elf-gcc'
compiler, commonly provided by Arch Linux and other distributions for
RISC-V embedded development.

The existing build system logic primarily checks for the
'riscv32-unknown-linux-gnu-gcc' and 'riscv64-unknown-elf-gcc'
toolchains.

The modified search priority is:

1. Risc-V 32 bit: `riscv32-unknown-linux-gnu-gcc`
2. Risc-V 64 bit: `riscv64-unknown-elf-gcc`
3. Risc-V (arch) 64 bit: `riscv64-elf-gcc`

I would also suggest that this order can be upgraded by searching for
the Risc-V 64 bit before searching for the 32 bit one.

**Notes:**
The Linux-targeting toolchain, 'riscv64-linux-gnu-gcc', is also
available on arch, further testing is needed to assess its suitability
versus the 'elf' variant for specific build configurations targeting
full operating systems.

Author: CaffeLatteIV

CMakeLists.txt

@@ -23,13 +23,17 @@ configure_file(config.h.in config.h)
 
 set(XT_PREFIXES ${RISCV_TOOL_PREFIX})
 if(NOT XT_PREFIXES)
-      find_program(GCC_TEST riscv32-unknown-linux-gnu-gcc)
-      if (GCC_TEST)
+      find_program(GCC_TEST_32 riscv32-unknown-linux-gnu-gcc)
+      find_program(GCC_TEST_64 riscv64-unknown-elf-gcc)
+      if (GCC_TEST_32)
         set(XT_PREFIXES riscv32-unknown-linux-gnu- 
           ${XT_PREFIXES})
-      else ()
+      elseif (GCC_TEST_64)
         set(XT_PREFIXES riscv64-unknown-elf- 
           ${XT_PREFIXES})
+      else ()
+        set(XT_PREFIXES riscv64-elf-
+        ${XT_PREFIXES})
       endif()
 endif()