Allows linker scripts to set the SEC_READONLY flag.

Allows linker scripts to set the SEC_READONLY flag.

* ld.texi: Document new output section type.
* ldgram.y: Add new token.
* ldlang.c: Handle the new flag.
* ldlang.h: Add readonly_section to list of section types.
* ldlex.l: Add a new identifier.
* testsuite/ld-scripts/output-section-types.t: New example linker script.
* testsuite/ld-scripts/output-section-types.d: Test driver.
* testsyute/ld-scripts/script.exp: Run the new test.