platforms: Add RISC-V 32-bit and 64-bit platform files

Refer to RISC-V calling conventions [1], this commit adds platform files
for RISC-V 32-bit and 64-bit plarforms.

[1]: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-cc.adoc#cc-type-sizes-and-alignments

Author: gagachang

platforms/riscv32.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0"?>
+<platform>
+  <char_bit>8</char_bit>
+  <default-sign>unsigned</default-sign>
+  <sizeof>
+    <bool>1</bool>
+    <short>2</short>
+    <int>4</int>
+    <long>4</long>
+    <long-long>8</long-long>
+    <float>4</float>
+    <double>8</double>
+    <long-double>16</long-double>
+    <pointer>4</pointer>
+    <size_t>4</size_t>
+    <wchar_t>4</wchar_t>
+  </sizeof>
+</platform>

platforms/riscv64.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0"?>
+<platform>
+  <char_bit>8</char_bit>
+  <default-sign>unsigned</default-sign>
+  <sizeof>
+    <bool>1</bool>
+    <short>2</short>
+    <int>4</int>
+    <long>8</long>
+    <long-long>8</long-long>
+    <float>4</float>
+    <double>8</double>
+    <long-double>16</long-double>
+    <pointer>8</pointer>
+    <size_t>8</size_t>
+    <wchar_t>4</wchar_t>
+  </sizeof>
+</platform>