GCC: remove -fno-delete-null-pointer-checks

For GCC we're being cautious by passing the
`-fno-delete-null-pointer-checks`. This option prevents some
optimisation opportunities, so removing it can reduce code size.

One particular optimisation loss occurs in `Callback` where a test
similar to this occurs:

    extern void myfunc();

    inline void foo(void (*fnptr)())
    {
        if (fnptr) {
            do A;
        } else {
            do B;
        }
    };

    foo(myfunc);

With `-fno-delete-null-pointer-checks`, the compiler does not assume
that `&myfunc` is non-null, and inserts the "null check" - seeing if the
address is 0. But performing that test of the address is incorrect
anyway - if myfunc actually could be at address 0, we'd still want to do
A.

Anyway, we do not have an equivalent option enabled for either Clang or
IAR, and we have performed clean-ups avoiding issues with
apparently-null vector tables in Clang already, for example #10534.

Therefore it should(TM) be safe to remove the option for GCC. We do not
have general data or code at address 0, only vectors are likely to be
there, so it does not make sense to be globally restricting code
generation for that.

tools/profiles/debug.json

@@ -4,7 +4,7 @@
                    "-Wno-unused-parameter", "-Wno-missing-field-initializers",
                    "-fmessage-length=0", "-fno-exceptions",
                    "-ffunction-sections", "-fdata-sections", "-funsigned-char",
-                   "-MMD", "-fno-delete-null-pointer-checks",
+                   "-MMD",
                    "-fomit-frame-pointer", "-Og", "-g3", "-DMBED_DEBUG",
                    "-DMBED_TRAP_ERRORS_ENABLED=1"],
         "asm": ["-x", "assembler-with-cpp"],

tools/profiles/develop.json

@@ -4,7 +4,7 @@
                    "-Wno-unused-parameter", "-Wno-missing-field-initializers",
                    "-fmessage-length=0", "-fno-exceptions",
                    "-ffunction-sections", "-fdata-sections", "-funsigned-char",
-                   "-MMD", "-fno-delete-null-pointer-checks",
+                   "-MMD",
                    "-fomit-frame-pointer", "-Os", "-g", "-DMBED_TRAP_ERRORS_ENABLED=1"],
         "asm": ["-x", "assembler-with-cpp"],
         "c": ["-std=gnu11"],

tools/profiles/release.json

@@ -4,7 +4,7 @@
                    "-Wno-unused-parameter", "-Wno-missing-field-initializers",
                    "-fmessage-length=0", "-fno-exceptions",
                    "-ffunction-sections", "-fdata-sections", "-funsigned-char",
-                   "-MMD", "-fno-delete-null-pointer-checks",
+                   "-MMD",
                    "-fomit-frame-pointer", "-Os", "-DNDEBUG", "-g"],
         "asm": ["-x", "assembler-with-cpp"],
         "c": ["-std=gnu11"],