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The Linux kernel uses -fpatchable-function-entry to implement DYNAMIC_FTRACE_WITH_REGS for arm64 and parisc. GCC 8 implemented -fpatchable-function-entry, which can be seen as a generalized form of -mnop-mcount. The N,M form (function entry points before the Mth NOP) is currently only used by parisc. This patch adds N,0 support to AArch64 codegen. N is represented as the function attribute "patchable-function-entry". We will use a different function attribute for M, if we decide to implement it. The patch reuses the existing patchable-function pass, and TargetOpcode::PATCHABLE_FUNCTION_ENTER which is currently used by XRay. When the integrated assembler is used, __patchable_function_entries will be created for each text section with the SHF_LINK_ORDER flag to prevent --gc-sections (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93197) and COMDAT (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93195) issues. Retrospectively, __patchable_function_entries should use a PC-relative relocation type to avoid the SHF_WRITE flag and dynamic relocations. "patchable-function-entry"'s interaction with Branch Target Identification is still unclear (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92424 for GCC discussions). Reviewed By: peter.smith Differential Revision: https://reviews.llvm.org/D72215