selftests/bpf: replace __regex macro with "{{...}}" patterns

Upcoming changes require a notation to specify regular expression
matches for regular verifier log messages, disassembly of BPF
instructions, disassembly of jited instructions.

Neither basic nor extended POSIX regular expressions w/o additional
escaping are good for this role because of wide use of special
characters in disassembly, for example:

    movq -0x10(%rbp), %rax  ;; () are special characters
    cmpq $0x21, %rax        ;; $ is a special character

    *(u64 *)(r10 -16) = r1  ;; * and () are special characters

This commit borrows syntax from LLVM's FileCheck utility.
It replaces __regex macro with ability to embed regular expressions
in __msg patters using "{{" "}}" pairs for escaping.
Syntax for __msg patterns:

    pattern := (<verbatim text> | regex)*
    regex := "{{" <posix extended regular expression> "}}"

For example, pattern "foo{{[0-9]+}}" matches strings like
"foo0", "foo007", etc.

Signed-off-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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