doc, riscv: Clean up RISC-V extensions documentation

doc, riscv: Clean up RISC-V extensions documentation

This patch fixes a number of problems I observed in the RISC-V
extensions documentation, which is autogenerated from .def files:

- The formatting of the table looked terrible in the PDF output, with
overlapping text.  I made the first two columns wider to fix this.

- Also the extension names in the table should have @samp{} markup.

- Many extensions were missing a full name/description.  (Documenting
something as "xyzzy extension" adds nothing useful to readers when we
are already listing the extension name "xyzzy" in the table.)

- Irregular spelling and capitalization in the full names.

gcc/ChangeLog
	* config/riscv/gen-riscv-ext-texi.cc: Fix table markup and
	layout.
	* config/riscv/riscv-ext-corev.def: Document missing extensions,
	regularize spelling/capitalization in existing descriptions
	* config/riscv/riscv-ext-mips.def: Likewise.
	* config/riscv/riscv-ext-sifive.def: Likewise.
	* config/riscv/riscv-ext-thead.def: Likewise.
	* config/riscv/riscv-ext.def: Likewise.
	* doc/riscv-ext.texi: Regenerated.