Support for -Z patchable-function-entry

`-Z patchable-function-entry` works like `-fpatchable-function-entry`
on clang/gcc. The arguments are total nop count and function offset.

See MCP rust-lang/compiler-team#704

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diff --git a/src/doc/unstable-book/src/compiler-flags/patchable-function-entry.md b/src/doc/unstable-book/src/compiler-flags/patchable-function-entry.md
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+# `patchable-function-entry`
+
+--------------------
+
+The `-Z patchable-function-entry=M,N` or `-Z patchable-function-entry=M`
+compiler flag enables nop padding of function entries with M nops, with
+an offset for the entry of the function at N nops. In the second form,
+N defaults to 0.
+
+As an illustrative example, `-Z patchable-function-entry=3,2` would produce:
+
+```
+nop
+nop
+function_label:
+nop
+//Actual function code begins here
+```
+
+This flag is used for hotpatching, especially in the Linux kernel. The flag
+arguments are modeled after hte `-fpatchable-function-entry` flag as defined
+for both [Clang](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-fpatchable-function-entry)
+and [gcc](https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html#index-fpatchable-function-entry)
+and is intended to provide the same effect.